Patients may be more familiar with the terms nettle rash or hives when talking about urticaria. This itchy rash results in skin oedema - the weal - often with erythema.

Weals are created by cells releasing histamine; however, other mediators, such as prostaglandins, are also released; this may in part explain why antihistamines do not always work in the management of urticaria.

The rash can change rapidly, sometimes daily, and may have resolved by the time the patient presents at the surgery. Acute urticaria is defined as lasting less than six weeks; after this it is termed chronic.

Angioedema and urticaria share the same causative factors but the pathological process lies deeper in the skin in the case of angioedema, which is associated with swelling as a prominent presentation.

1. Aetiology and pathogenesisA number of causal factors can be implicated in urticaria, such as food, medication (such as ACE inhibitors and NSAIDs) and infection. The diversity of potential causes makes detailed clinical investigations difficult and these may not be indicated in an acute episode.

The pathogenesis is complex and involves degranulation of mast cells, with release of histamine, leukotrienes and other products. Vascular permeability alters, allowing fluid to seep into adjacent tissues. Autoimmune factors, complement and bradykinins may also be involved.

Urticaria is common, with a significant proportion of the population having an episode at some point in their life; however, many episodes are brief.

2. Clinical featuresUrticarial weals can appear over a period of just a few minutes. Individual lesions may only persist for several hours, but can reappear in other sites and vary in size and shape. Lesions do not scale, but blanch on pressure and are often raised and red, and can be very itchy.

When the skin oedema affects the dermal layer or deeper, it is known as angioedema. This can affect the face and hands, but is normally not associated with pruritus in the same way as urticaria.

However, angioedema can affect the airway and compromise its patency; though serious and potentially life-threatening, this is rare. Both urticaria and angioedema can affect any skin surface, either in combination or separately.

Sometimes urticaria is induced by a specific physical cause, for example direct pressure, cold or heat and sunlight (solar urticaria), or contact with chemicals or water (aquagenic urticaria).

Dermatographism can be caused when the skin comes into contact with a firm stimulus that is dragged across its surface. The weal appears shortly after the stimulus is applied and can appear as a line of urticaria following the direction of the pressure. It is not uncommon and the pressure does not have to be that intense to produce the effect.

Cholinergic urticaria tends to affect younger people. Affected areas are much smaller and can be associated with raised body temperature produced by fever or exercise. The lesions can be itchy.

Because of the short-lived nature of urticarial lesions, marking their edges with a pen may help when following up the patient over the next day or so.

3. InvestigationsThe first, most important investigation is to take a good, detailed history. Considering the diverse causes of urticaria, this may take time and sometimes the patient will have no rash to demonstrate, although they may have taken photographs of active lesions on their mobile phone; it may be worth asking them to do this if they have not already done so.

If the history is suggestive of a physical cause, this may be reproduced during a physical examination. However, this should be done sensitively and sensibly.

Many people with chronic urticaria will have no obvious abnormality.

Routine baseline investigations, such as basic haematology and biochemistry, are not indicated at the outset, when the diagnosis may be made clinically, unless there is something significant in the history. However, these investigations may be required if the condition becomes chronic and intrusive for the patient.

Key points

Urticaria is common and can be acute (less than 6 weeks or chronic >6 weeks).

An urticarial rash can change rapidly, some lesions resolving within hours.

Taking a good, detailed history is vital.

Provide patients with a full explanation of their condition.

Not all cases will require treatment, but ice packs and antihistamines may provide symptomatic relief.

Consider referral if the urticaria becomes chronic.

4. TreatmentIf patients seek medical advice for urticaria they should receive a full explanation and description of the problem. Reassurance may be all that is required.

Obviously any underlying cause should be treated if appropriate or possible, but not all cases need treatment and low-grade episodes may be left to settle by themselves.

Short-term symptomatic relief may be obtained by applying an ice pack to the affected area for a short period.

If pharmacological intervention is required, an antihistamine (H1 blocker) can be used.

The sedating type may be useful taken at night in order to help improve sleep, if required.

An H2 blocker (for example, cimetidine) can be added to the H1 blocker, but this combination is not often successful. Occasionally systemic steroids may be used for their potent anti-inflammatory properties; however, ongoing use is limited by significant side- effects.

If basic measures do not control the condition or the urticaria becomes chronic, consider referral to a dermatology unit.